1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu or Ag. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
The CPP MR sensors described above are “single” CPP sensors because they have a single spacer layer and a single reference layer. A “dual” CPP MR sensor has a second spacer layer and a second reference layer located on the other side of the free layer, i.e., the side opposite the first spacer layer and first reference layer. Dual CPP MR sensors are well-known in the art and have been proposed to provide an increased magnetoresistance (ΔR/R) over single CPP MR sensors. Single CPP MR sensors are known to be susceptible to current-induced noise and instability. The spin-polarized current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque on the local magnetization. This can produce continuous gyrations of the magnetization, resulting in substantial magnetic noise if the sense current is above a certain level. It has been demonstrated that dual CPP-GMR sensors may reduce the sensitivity of the free layer to spin-torque-induced instability. (J. R. Childress et al., “Dual current-perpendicular-to-plane giant magnetoresistive sensors for magnetic recording heads with reduced sensitivity to spin-torque-induced noise”, J. Appl. Phys. Vol. 99, 08S305, 2006).
The sensor stack in a CPP MR read head is located between two shields of magnetically permeable material that shield the read head from recorded data bits that are neighboring the data bit being read. As the data density increases in magnetic recording disk drives, the read head dimensions decrease. In particular, the flux from the neighboring bits in the along-the-track direction needs to be shielded in to improve the spatial resolution of the sensor. This requires that the sensor stack be thinner, and also that the shield-to-shield spacing be reduced. However, it becomes difficult to reduce the thickness of the free and reference ferromagnetic layer without a significant reduction in the sensed signal. It is also difficult to reduce the thickness of the antiferromagnetic layer without losing magnetic stability in the reference layer.
What is needed is a CPP MR sensor with significantly reduced shield-to-shield spacing that does not suffer from reduced signal reduction or reduced magnetic stability.